1. Field of the Invention
The present invention relates to a photoelectric converter, a method for driving the converter and a system thereof, for example, to a one-dimensional or two-dimensional photoelectric converter capable of reading data of a facsimile machine, a digital copying machine, an X-ray photographing device or the like, a method for driving the converter and a system thereof.
2. Related Background Art
Conventionally, a reading system using a reducing optical system and a CCD sensor has been used as a reading system for a facsimile machine, a digital copying machine, an X-ray photographing device or the like. In recent years, with the development of photoelectric conversion semiconductor materials represented by hydrogenated amorphous silicon (hereinafter represented as "a-Si"), the development of so-called closely laminated type sensors for reading data with an optical system of the same size as the information source by forming a photoelectric conversion element and a signal processing part on a large-area substrate is being targeted. In particular, since a-Si can be used not only as a photoelectric conversion material but also as a thin-film electric field effect type transistor (hereinafter described as a "TFT"), there is an advantage that a photoelectric conversion semiconductor layer and a semiconductor layer for the TFT can be formed at the same time.
FIGS. 1 and 2 are schematic sectional views showing one example of a structure of an optical sensor, respectively. FIG. 1 and FIG. 2 are views schematically showing a structure of a layer of the optical sensor. FIG. 3 is a view showing one example of a representative driving method which is common among optical sensors. FIG. 1 and FIG. 2 are views both showing a photo-diode type optical sensor. FIG. 1 is a view showing a so-called PIN type optical sensor while FIG. 2 is a view showing a so-called Schottky type optical sensor. In FIGS. 1 and 2, reference numeral 1 denotes a substrate having at least an insulated surface, 2 a lower electrode, 3 a p-type semiconductor layer (hereinafter referred to as a "p layer"), 4 an intrinsic semiconductor layer (hereinafter referred to as an "i layer"), 5 an n-type semiconductor layer (hereinafter referred to as an "n layer"), and 6 a transparent electrode. In a Schottky type optical sensor shown in FIG. 2, the material of a lower electrode 2 is appropriately selected so that a Schottky barrier layer is formed to inhibit the injection of electrons from the lower electrode 2 into the i layer 4. In FIG. 3, reference numeral 10 shows an optical sensor which is represented by symbolizing the above optical sensor. Reference numeral 11 denotes a power source. Reference numeral 12 denotes a detecting element of a current amplifier or the like. The direction shown by symbol C in the optical sensor 10 is directed toward the side of a transparent electrode 6 in FIGS. 1 and 2 while the direction denoted by symbol A in the optical sensor 10 is directed to the side of the lower electrode 2, and the power source 11 is set up so that a positive voltage is applied to side C with respect to side A.
Operation of the optical sensor will be briefly explained hereinafter. As shown in FIGS. 1 and 2, light is entered from the direction shown by an arrow. When the light reaches the i layer 4, the light is absorbed and electrons and holes are generated. Since an electric field is applied to the i layer 4 by the power source 11, electrons are moved to the side C, namely to the transparent electrode 6 via the n layer 5 while holes move to side A, namely, to the lower electrode 2. Consequently, a photoelectric current flows through the optical sensor 10. When light is not entered, electrons and holes are not generated in the i layer 4. Furthermore, since an n layer 5 serves as a hole injection blocking layer, the p layer 3 of the PIN type shown in FIG. 1 and the Schottky barrier layer in the Schottky type shown in FIG. 2 serves as an electron injection blocking layer, so that the holes in the transparent electrode 6 and the electrons in the lower electrode 2 cannot be moved, respectively and current does not flow. Consequently, the current is changed depending on the presence or absence of light entrance. When a change in current is detected by the detecting element 12 of FIG. 3, the optical sensor is operated.
However, it is not easy to actually produce at a low cost a photoelectric converter having a sufficient SN ratio using the conventional optical sensor. The reasons will be explained hereinafter.
A first reason is that both the PIN type shown in FIG. 1 and the Schottky type shown in FIG. 2 require an injection blocking layer at two places.
In the PIN type sensor, the n layer 5, which is an injection blocking layer, blocks the movement of electrons to the transparent electrode 6. At the same time, a property to block the injection of holes into the i layer 4 is needed. Furthermore, in the Schottky type sensor, the Schottky barrier layer requires properties for blocking electrons from the lower electrode and for blocking holes from the n layer 5. When either of these properties is lost, the photoelectric current is lowered, and current generated without incident light (hereinafter referred to as "dark current") increases, which causes a decrease in the SN ratio. This dark current itself is considered to be noise. At the same time, a fluctuation referred to as shot noise, or so-called quantum noise, is included. Even when the detecting element 12 performs a process to subtract the dark current, the quantum noise that accompanies the dark current cannot be reduced. Normally, in order to improve this property, it is required that the film formation conditions for i layer 4 and n layer 5 and the annealing conditions after the preparation of the film be optimized. However, with respect to the p layer 3, which is another injection blocking layer, the same property is required even though the electrons and holes are the opposite. In the same manner, optimization of each condition is required. Normally, the conditions for the optimization of the former n layer and for the optimization of the latter p layer are not the same. It is not easy to satisfy both conditions at the same time. In other words, injection blocking layers having two properties opposite to each other at two places are needed in the same optical sensor, making it difficult to form an optical sensor having a high SN ratio. This is applied to the Schottky type shown in FIG. 2. In the Schottky type shown in FIG. 2, the Schottky barrier layer is used as one of the injection blocking layers. This utilizes the difference between the work functions of the lower electrode 2 and the i layer 4. The material of the lower electrode 2 is restricted, and the influence of the localized level of the interface largely affects the properties. Consequently, it is not easy to satisfy all the conditions in an ideal manner. Furthermore, in order to improve the properties of the Schottky barrier layer, it has been reported that a thin oxide film of silicon or a metal or a nitride film having a thickness of about 100 angstroms or the like may be formed between the lower electrode 2 and the i layer 4. This is intended to use a tunnel effect, introduce holes to the lower electrode 2, and block the injection of electrons to the i layer 4. The difference in work functions is also used here. For this purpose, limitation of the material of the lower electrode 2 is required. Furthermore, the thicknesses of the oxide film and nitride film are restricted to a very thin level of about 100 angstroms because of the use of opposite properties such as the block of the injection of electrons and hole movement caused by the tunnel effect. Furthermore, it is difficult to control the thickness or the film quality, and it is not easy to raise productivity.
In addition, the fact that an injection blocking layer is needed at two places lowers productivity, constituting a factor for cost increases. This is because the injection blocking layer is important in terms of properties, with the result that when a defect is generated by dust or the like at one of the two places, the properties of an optical sensor cannot be obtained.
The second reason will be explained by referring to FIG. 4. FIG. 4 is a view showing a layer structure of an electric field effect type transistor (TFT) formed by thin film semiconductor layers. The TFT may be used as a part of a control part for forming a photoelectric converter. In FIG. 4, the same constituent parts are denoted by the same reference numeral in FIGS. 1 and 2. In FIG. 4, reference numeral 7 denotes a gate insulation film. Reference numeral 60 denotes an upper electrode. The method for forming the TFT will be explained in due order. On the insulation substrate 1, a lower electrode 2 which serves as a gate iselectrode, the gate insulation film 7, an i layer 4, an n layer 5, and the upper electrode 60 which serves as a source and drain electrode are formed in this order. The source and drain electrodes are formed by etching the upper electrode 60. Thereafter, a channel part is constituted by etching the n layer 5. The property of the TFT is very sensitive to the state of the interface between the gate insulation film 7 and the i layer 4. Therefore, the insulation film 7 and the i layer 4 are normally deposited in a continuous manner under the same vacuum in order to prevent contamination.
When an optical sensor having a layer structure shown in FIGS. 1 and 2 is formed on the same substrate as this TFT, the layer structure of the TFT has a problem that may induce cost increases and property deterioration. The reason for this is as follows. With the PIN type optical sensor shown in FIG. 1, the optical sensor is constituted in such a manner that the electrode/the p layer/the i layer/the n layer/and the electrode are formed in this order from the side of the substrate. With the Schottky type optical sensor shown in FIG. 2, the electrode/the i layer/the n layer/and the electrode are formed in this order from the side of the substrate. On the other hand, with the TFT, the electrode/the insulation film/the i layer/the n layer/and the electrode are formed in this order from the side of the substrate. Thus, the conventional optical sensors and the TFT are different in their constitution. This means that the each films cannot be formed in the same process. In other words, yield is lowered and cost increases because of the complicated nature of the process. In order to form the i layer/n layer at the same time with a common process, a process for etching the gate insulation layer 7 and the p layer 3 is needed. This means a possibility that the p layer 3 and the i layer 4, which constitute an injection blocking layer, one of the important layers of the aforementioned optical sensor, cannot be formed in a continuous manner under the same vacuum, and the interface between the gate insulation film 7 and the i layer 4, which are important for the TFT, will possibly be contaminated by the etching for the patterning of the gate insulation film, as well as a factor for property deterioration or a decrease in the SN ratio.
Furthermore, in order to improve the property of the Schottky type sensor shown in FIG. 2, the order of the film structure may be made the same in the case of forming an oxide film or a nitride film between the lower electrode 2 and the i layer 4. However, as described before, it is required that the thickness of the oxide film and the nitride film be set at about 100 angstroms. The oxide film or the nitride film cannot be substantially used in common with the gate insulation film 7. FIG. 5 is a graph showing one example of our experiment results on the gate insulation film 7 and yield of the TFT. When the thickness of the gate insulation film is 1000 angstroms or less, the yield abruptly lowers. When the thickness is 800 angstroms, the yield is about 30%. When the thickness is 500 angstroms, the yield is 0%. When the thickness is 250 angstroms, even the operation of the TFT cannot be confirmed. In this way, it is normally difficult to commonly use the oxide film or the nitride film of an optical sensor using the tunnel effect and the gate insulation film of a TFT in which electrons and holes must be insulated.
Furthermore, it is difficult to prepare a capacity element (hereinafter referred to as a "capacitor") having favorable properties with a small amount of leakage in the same structure as the optical sensor. The capacity element constitutes an element needed for obtaining an integral amount of electric charge and current (not shown). The capacitor is intended to accumulate electric charge between the two electrodes, so that a layer for blocking movement of electrons and holes needed in an intermediate layer between the electrodes. On the other hand, with a conventional optical sensor, it is difficult to obtain an intermediate layer with favorable properties such as heat stability and low leakage because only a semiconductor layer or a layer in which either electrons or holes move is provided between the electrodes.
In this way, the matching is not favorable in either process or properties for the TFT and capacitor that are important elements constituting a photoelectric converter. Thus the processes become numerous and complicated for forming an entire system that a plurality of optical sensors are arranged one-dimensionally or two-dimensionally and successively detects light signals, so that it is not easy to raise yield. There is yet room for improvement in preparation of a high performance device with many functions at low cost.